1. Field
The invention relates to the field of radiation therapy and to systems and methods for analyzing interactions of protons with tissue or other materials for imaging and to improve diagnosis and accuracy of therapy delivery.
2. Description of the Related Art
Proton beam therapy has known and potential benefits in treatment of a wide variety of disease conditions. Protons at certain energies exhibit a useful characteristic of a relatively high degree of controllability and selective transfer of energy to target tissue with relatively low undesired transfer of energy to non-target tissue. Protons exhibit the physical characteristic of a Bragg peak where a substantial fraction of the energy of a beam of accelerated protons is delivered within a relatively narrow penetration depth and where the depth can be selected and controlled based on the characteristics of material through which the proton beam passes and the energy of the protons. The highest energy deposition per unit length (LET) is typically exhibited at the end of range of such a proton beam, e.g. at the Bragg peak, and this also corresponds to the region of maximum absorbed dose. The characteristics of protons result in a relatively low entrance dose to non-target tissue upstream of the target region and a relatively low (approaching negligible) exit dose (e.g. to non-target tissue downstream from the target region with proper selection of beam energies).
This feature allows a clinician to adjust the proton energy such that the depth of the Bragg peak coincides with the spatial location of target tissue. In many applications, a collimator is used to control the focus of the proton beam. A focused proton beam can be raster scanned and/or modulated to deliver a selected radiation dose to a distributed target region with significantly reduced undesired transfer of energy to non-target tissues, for example as occurs with photonic radiation therapy.
It will be understood however that calculation of an appropriate proton dose and selection of beam energy to achieve a desired depth or range is dependent on accurate knowledge of the characteristics of the materials through which the beam will pass. In some implementations, x-ray imaging and/or computed tomography (CT) is utilized to obtain indications of the internal structures and compositions of the patient, including the intended target region of the proton therapy and intervening non-target tissue. However, inaccuracies and/or uncertainties can arise in images that are based on electron density distributions, thereby leading to corresponding uncertainties in dose and proton range values.